The present invention relates to fluid flow meters and, in particular, discloses an ultrasonic fluid flow meter which provides accurate timed reception of an ultrasonic pulse.
Ultrasonic fluid meters are used for the measurement of fluid flow parameters, typically of fluids such as hydrocarbon gas and/or petroleum. Recent developments in this technological field have indicated that in order to obtain high levels of accuracy, a number of the aspects of operation of an ultrasonic flow meter must be optimised. For example, the ring-around (or sing-around) technique of transmission of ultrasonic pulses is known to provide an accurate fluid flow measurement obtained through averaging propagation times. From such an average, fluid flow parameters can be readily calculated. However, when using this ring-around technique, the detection of the time of receipt of the ultrasonic signal is critical and becomes difficult due to the complex interaction of high order acoustic modes which propagate together with a plane wave (or fundamental) mode in the measuring tube. Notably, certain high order acoustic modes typically travel at speeds close to integral fractions of the plane wave mode to the extent that high order modes from one acoustic wave packet interfere with the plane wave modes of a number of following acoustic wave packets in the ring-around sequence.
There have been proposed methods of detecting the arrival of the acoustic wave packet based upon an assessment of the amplitude of the received acoustic wave packet and identifying a specific reference (eg. zero) level crossing of the wave packet which can be used to trigger the transmission of the next wave packet in the ring-around sequence. However, the present inventors have determined that the use of amplitude techniques alone is insufficient to provide for sufficiently accurate detection of the acoustic wave packet. This is because the received amplitude can be often corrupted by noise and high order acoustic modes which can cause errors that are unacceptable at the desired levels of precision.
It is an object of the present invention to address these problems of detection of an ultrasonic acoustic wave in a fluid flow meter.
In accordance with one aspect of the present invention there is disclosed a method of detecting the arrival at a transducer of an acoustic wave packet within a series of acoustic wave packets, said method comprising the steps of:
(a) determining a nominal time of propagation of a single acoustic wave packet by:
(i) simultaneously starting a timer and transmitting said single acoustic wave packet to said transducer;
(ii) receiving said single acoustic wave packet at said transducer; and
(iii) identifying a predetermined part of said wave packet and stopping said timer to thereby determine said nominal time of propagation;
(b) transmitting said series of acoustic wave packets to said transducer wherein the transmission of any one wave packet within said series, excepting a first wave packet, is initiated upon detection of arrival at said transducer of an immediately preceding wave packet in said series;
(c) upon each transmission of said wave packet in step (b) starting a timer; and
(d) determining when said timer is within a predetermined time of said nominal time of propagation to then enable identification at said transducer of said predetermined part of the corresponding wave packet to thereby detect the arrival of said corresponding wave packet.
In accordance with a another aspect of the present invention there is disclosed a method of detecting the arrival at a transducer of an acoustic wave packet within a series of acoustic wave packets, where the transmission of any one wave packet in said series, excepting a first wave packet, is initiated upon detection of arrival at said transducer of an immediately preceding wave packet in said series, and the detection involves identifying a particular part of a waveform received at said transducer after arming a receiving arrangement that detection is imminent, characterised in that said method includes, prior to transmission of said series, transmitting a single acoustic wave packet to said transducer and determining a nominal propagation time of said single wave packet by identifying a corresponding particular part of a corresponding waveform received without arming said receiving arrangement, whereby said nominal propagation time is used to arm said receiving arrangement for the detection of each said wave packet within said series.
In accordance with another aspect of the present invention there is disclosed an electronic fluid meter comprising:
a duct through which a fluid may flow;
at least two acoustic transducers arranged within said duct to transmit acoustic energy therebetween, said transducers being separated by a predetermined distance defining a measurement portion therebetween;
a control system for causing transmission and monitoring reception of acoustic wave packets between said transducers, said control system including:
first means for determining a nominal time of propagation of a single acoustic wave packet, said first means including:
(i) second means for simultaneously starting a timer and transmitting said single acoustic wave packet to said transducer;
(ii) third means for receiving said single acoustic wave packet at said transducer; and
(iii) fourth means for identifying a predetermined part of said wave packet and stopping said timer to thereby determine said nominal time of propagation;
fifth means for transmitting said series of acoustic wave packets to said transducer wherein the transmission of any one wave packet within said series, excepting a first wave packet, is initiated upon detection of arrival of an immediately preceding wave packet in said series;
sixth means for, upon each transmission of said wave packet, starting a timer; and
seventh means for determining when said timer is within a predetermined time of said nominal time of propagation to then enable identification of said predetermined part of the corresponding wave packet to thereby detect the arrival of said corresponding wave packet.
In accordance with another aspect of the present invention there is disclosed a method of measuring a fluid flow parameter, said method comprising the steps of:
transmitting an ultrasonic test pulse from a first location within a fluid flow path,
receiving said ultrasonic test pulse at a second location within the fluid flow path to determine a time delay value between a start of transmission of said test pulse and an active zero crossing of said received ultrasonic test pulse,
thereafter transmitting from said first location, a plurality of successive ultrasonic measurement pulses which are received at said second location by crossing a zero reference value after expiration of said time delay value being started simultaneously with transmission of each of said successive ultrasonic measurement pulses.